In the oil and gas exploration and production industry, well boreholes are drilled in order to access subsurface hydrocarbon-bearing formations. The drilled borehole may then be lined with sections of bore-lining tubing, such as casing or liner. In some instances, each section of bore-lining tubing may be provided with threaded connectors, or otherwise joined, to form a string, such as a completion string, which is run into the borehole and operable to perform a number of different operations in the borehole. One operation which may be carried out in the borehole is hydraulic fracturing, commonly known as “fracking”, which involves the injection of fluid into the formation to propagate fractures in the formation rock and increase flow of hydrocarbons into the borehole for extraction. In use, one or more fracturing tools may be run into the borehole with the completion string and located adjacent to the formation. Fluid may then be directed through ports in a sidewall of the fracturing tool and injected into the formation. In some instances, a number of fracturing tools may be located at different axially spaced positions in the completion string and configured to facilitate fracturing of multiple and/or selected formations.
Completion strings are becoming ever more complex, with the various completion string tools utilising a variety of activation mechanisms, forces and pressures. Also, completion strings may in many instances be run in non-vertical, horizontal or deviated boreholes in which the distal end, or toe, of the borehole may be a significant lateral distance away from the wellhead.
The increased complexity of completion strings, and the complex geometry and topography of some boreholes may present a number of problems.
For example, in deviated or horizontal boreholes the ability to apply and control application of mechanical forces to a given tool of the completion string, such as to activate and/or deactivate the tool, may be limited. Where it is desired to apply a push or pull force to activate a tool of the completion string, for example, it will be recognised that for horizontal or deviated boreholes the vertical proportion of the completion string to which the push or pull force is applied may be relatively small. As a result, accurate control of the greater proportion of the completion string disposed in the non-vertical section of the borehole is limited.
Fluid pressure activation arrangements may permit tools to be controlled over distance and in both vertical and non-vertical borehole sections. However, there is a risk that a given tool may activate prematurely in complex completion strings having a number of fluid pressure activated tools operable at a variety of activation pressures. In some instances, such premature activation may reduce the efficiency of hydrocarbon extraction from the borehole. However, in other instances premature activation of a tool may require the completion string to be removed, where this is possible, a workover operation to be carried out, or may even result in the borehole being abandoned, at significant time and expense to the operator.